Even though current technologies of fiber laser have made significant progress toward achieving a compact and reliable fiber laser system providing high quality output laser with ever increasing output energy, however those of ordinary skill in the art are still confronted with technical limitations and difficulties. Specifically, when a short pulse high power laser is transmitted through a length of fiber and several stages of amplifiers, the pulse is distorted due to nonlinear effects (such as self phase modulation (SPM), gain narrowing effect GNE, etc., and the pulse width usually becomes broadened. The broadened pulse width thus increases the difficulties of generating the high-energy laser of short pulse lasers, e.g., a laser with a femtosecond pulse width.
Furthermore, in a fiber laser system implemented with the Chirped Pulse Amplification (CPA) for short pulse high power laser amplifier, the CPA systems are still limited by the technical difficulties that the third order dispersion (TOD) limits the scalability of the laser systems. Such limitations were not addressed in the conventional technologies due to the fact that the conventional solid-state laser utilizes Grating-Lens combination and Treacy compressor for pulse stretching and compressing. Ideally, in such solid-state systems, all orders of dispersion can be compensated, but the material dispersion can distort and damage this ideal situation. But the material dispersion is not a serious problem in solid-state laser system because the material dispersion is generally considered as not important. However, for a fiber laser system, the situation is different due to the fact that in the fiber laser systems, attempts are made by using the fiber stretcher to replace the grating-lens combination for the purpose of significantly increasing the system reliability. However, the TOD limits the ability for de-chirping when using Treacy compressor since both fiber stretcher and Treacy compressor have positive TOD even this combination can remove the second order dispersion completely. This issue of TOD dispersion makes it more difficult to develop a high-energy fiber laser amplifier with <200 fs pulse width. Actually, the technical difficulty of TOD dispersion is even more pronounced for laser system of higher energy. A laser system of higher energy requires a higher stretch ratio and that leads to a higher TOD. Therefore, for laser system of higher energy, it is even more difficult to re-compress the pulse to the original pulse width.
A chirped Pulse Amplification (CPA) is widely implemented with four parts: a mode-locking (ML) oscillator providing short pulse, a stretcher to get long pulse duration, an amplifier to get high energy, and a compressor to get short pulse and high peak power. In order to obtain a very high peak power, it is required to obtain a high energy and short pulse width at the same time. However, the issue of generating a short pulse width in a fiber laser system is always difficult to achieve due to an uncompensated highly positive third-order dispersion (TOD) generally referred to as the compressibility issue. For the purpose of resolving this compressibility issue, a number of ideas, including new compressor designi and new stretcher are disclosed in different patent applications including patent applications Ser. No. 06/062,205 and Ser. No. 06/062,905 and the disclosures are hereby incorporated as reference in this Patent Application. In the application Ser. No. 06/062,906, the self-phase modulation (SPM) was intentionally created and utilized in the stretcher stage in order to reduce the stretcher fiber length and the TOD impact. In addition to above disclosures made by the Applicant of this invention, further improvements to the laser system are still required to resolve this compressibility issue.
In the conventional laser system, the elimination of all nonlinear effects is generally considered as a desirable goal in the conventional design of the high-energy fiber laser system, It was widely believed that the nonlinear effects will degrade the pulse quality and stability. However, in practice, this conventional dogmatic design concept is no longer considered as universally acceptable. A specific example is the use of the Stimulated Raman Scattering (SRS) effects in a fiber Raman amplifier, and nonlinear phase shift accumulated in the amplifier stage was implemented to compensate the TOD. However, in the CPA fiber laser system, the SRS scattering must be avoided. The concept of applying the phase shift and spectral generation for improvement of pulse quality as that disclosed in a fiber Raman amplifier does not provide a direct solution to the difficulties caused by the compressibility issues due to the technical challenges arise from the third order dispersion effects.
Therefore, a need still exists in the art of fiber laser design and manufacture to provide a new and improved configuration and method to provide fiber laser to overcome the gain narrowing effect in amplifying the laser pulses such that the above-discussed difficulty may be resolved.